I was trying to decide the same thing and ended up going with 19". The benefits of 18" are that they are cheaper and you will be able to replace the tires for less $$ and the additional sidewall for protection.

If I would have gotten 18" I would have regretted it. 19" just looks so much better.

I'm running 19's now, but sure miss the 18's I had on my saloon. In my opinion, aside from appearance, 18's are better in every way, including weight, comfort, ride quality, wheel protection, and braking. If I replace the stockers on my current ride, I'll definitely consider returning to 18's. The only reason to run 19's on our cars is looks.